Lipids
The term lipid refers to a diverse range of molecules and to some extent it is a catch-all for relatively water-insoluble or nonpolar compounds of biological origin, including waxes, fatty acids, fatty-acid derived phospholipids, sphingolipids, glycolipids, and terpenoids such as retinoids and steroids. Some lipids are linear aliphatic molecules, while others have ring structures. Some are aromatic, while others are not. Some are flexible, while others are rigid.
Phospholipids are a special group of lipids containing phosphate. Phospholipids are the building blocks of liposomes and cell membranes. Human skin, like the rest of the human body, is composed of cells whose membranes must be healthy and strong in order for them to function properly.
Lipids in general are hydrophobic, also called non-polar (not able to be mixed in water). However, the phosphate group in phospholipids is hydrophilic, also called polar (able to be mixed in water).
When phospholipids are immersed in water they arrange themselves so that their hydrophilic regions point toward the water and their hydrophobic regions point away from the water. This unique simultaneously hydrophilic/hydrophobic structure of phospholipids is the key to their ability to organize as a double layer (bilayer formation) when immersed in water. The interaction and rejection forces between phospholipids and water cause phospholipids to organize themselves as bilayers. Phospholipid bilayers are the core structure of liposome and cell membrane formations. A very small liposome (single-bilayer) is referred to as a nanosome.
Merely by mixing lipid (oil) with water, one can create liposome structures in single layered vesicles (SV) or multi-layered vesicles (MLVs). The physical properties of lipid and water yield a spherical shaped vesicle.
Cholesteryl Esters
A sterol alcohol, e.g., cholesterol, can replace one of the triglycerides of a fatty acid lipid to become a lipid sterol. A sterol alcohol, e.g., cholesterol can also become esterified to become a cholesteryl ester. Unlike a lipid, this cholesteryl ester has only one tail.
Cholesterol has the structural formula:
The H atoms are not shown in the structural formula for clarity.
Fatty acid esters of cholesterol constitute about two-thirds of the cholesterol in the plasma. Cholesterol is the main sterol present in animal tissues but other sterols may be present in biological extracts prepared from vegetables.
Cholesteryl esters, i.e., with long-chain fatty acids linked to the hydroxyl group, are much less polar than free cholesterol and appear to be the preferred form for transport in plasma and for storage. Those in plasma are synthesised largely by transfer of fatty acids from position sn-2 of phosphatidylcholine by the enzyme lecithin-cholesteryl acyl transferase (LCAT). Cholesteryl esters are major constituents of the adrenal glands, where esters of steroidal hormones, may also be present at low levels. The latter may be a biologically inert storage form. Esters with long chain fatty acids are a common form of transport and storage of cholesterol. Cholesteryl linoleate is one of the most abundant cholesteryl ester species in human plasma.
Cholesteryl esters are prevalent in animal body fluids as plasma lipoproteins, and may be found in vessel walls as fatty streaks in atherosclerosis. The accumulation of cholesteryl esters in the arterial intima is a characteristic feature of atherosclerosis. Acylated sterols are also found in plant structures, their characteristics are similar to those of cholesteryl esters.
Lipids and proteins can form monolayers on both extra and intracellular leaflets of bilayer membranes, as well as other assemblies such as lipoproteins and micelles. Interfacial binding is an important mode of interaction for many lipid enzymes as well as effectors of lipids. It is governed by electrostatics at the interfacial region and the characteristics of lipids head groups. The interior portion of a lipid assembly, e.g., the bilayer interior, contributes with interactions that arise from the hydrophobic parts of lipid molecules that play a role in the regulation of membrane channels for example.